The present invention relates to an optical amplifier with an erbium-doped fiber or, more particularly, to an optical amplifier with an erbium-doped fiber of which the amplification gain characteristics can be optimized.
It is well known that a single-mode erbium-doped fiber can be used in optical amplifiers. The block diagram of FIG. 1 shows constitution of a typical optical amplifier of the backward pumping type. In this assembly, the signal power of, for example, 1.55 .mu.m wavelength entering the isolator 1 for 1.55 .mu.m wavelength and incident to the erbium-doped fiber 2 is introduced into the WDM (wavelength division multiplexer) coupler 6, along with excitation of laser emission by the pumping light 4 of 1.48 .mu.m wavelength coming from the LD driver 3 and passing through the isolator 5 for 1.48 .mu.m wavelength, from which the amplified signal power of 1.55 .mu.m wavelength is outputted through the second isolator 7 for 1.55 .mu.m wavelength. It is also known that, in an optical amplifier with an erbium-doped fiber in the 1.55 .mu.m range of wavelength in the optical communication system, the gain characteristics thereof depend on the length of the optical fiber so that the highest gain is obtained only with an optimum length of the optical fiber. The optimum length of the optical fiber varies from 10 meters or shorter to 200 meters or longer depending on the doping level of the erbium dopant in the core of the optical fiber and structure of the optical fiber.
From the practical standpoint for optical amplification, it is important that the optimum length of the optical fiber is desirably within a certain range because, when the erbium-doped fiber for the optical amplifier is too long, the housing of the instrument also must undesirably be large enough in order to contain the so long optical fiber therein while, when the length of the optical fiber is too short, the gain may be subject to great variations even with a relatively small change in the length unavoidably caused due to the loss by cutting at the fiber terminals for the connection thereof to an optical amplifier. This is the reason for the requirement that an erbium-doped fiber for an optical amplifier is designed to have an optimum length of the optical fiber which falls within an appropriate range depending on the particular applications.